As you might expect, Libbrecht hears the question only too often. But the answer is not so simple: It depends on what you mean by the word alike, Libbrecht says. It's essentially impossible that any two visible ice crystals would have exactly the same molecular structure, but small, simple, hexagonal prism crystals might look alike to the naked eye.
But large, complex snowflakes are a different story. With the staggering number of variables that affect each step of crystal growth, Libbrecht says he's willing to back the conventional wisdom. Out of all the complex snow crystals ever made over the history of the planet, it's unlikely that any two ever looked completely alike, he says.